This invention relates to the roughening of smooth planar surfaces such as metal or plastic and especially to the preparation of a grained aluminum base for lithographic printing plates, and more particularly to an anodized aluminum base having improved abrasion resistance and long press-life.
The art of lithographic printing depends upon the immiscibility of grease and water, upon the preferential retention of a greasy image-forming substance by an image area, and upon the similar retention of an aqueous dampening fluid by a non-image area. When a greasy image is imprinted upon a suitable surface and the entire surface is then moistened with an aqueous solution, the image area will repel the water and the non-image area will retain the water. Upon subsequent application of greasy ink, the image portion retains ink whereas the moistened non-image area repels it. The ink on the image area is then transferred to the surface of a material on which the image is to be reproduced, such as paper, cloth and the like, via an intermediary, a so-called offset or blanket cylinder, which is necessary to prevent mirror-image printing.
The type of lithographic plate to which the present invention is directed has a coating of a light-sensitive substance that is adherent to an aluminum base sheet. If the light-sensitive coating is applied to the base sheet by the manufacturer, the plate is referred to as a "presensitized plate". If the light-sensitive substance is applied to the base by the lithographer or trade plate-maker, the plate is referred to as a "wipe-on" plate. Depending upon the nature of the photosensitive coating employed, the treated plate may be utilized to reproduce directly the image to which it is exposed, in which case it is termed a positive-acting plate, or to produce an image complementary to the one to which it is exposed, in which case it is termed a negative-acting plate. In either case the image area of the developed plate is oleophilic and the non-image area is hydrophilic.
In the case of a negative working plate the surface is coated with an aqueous solution of a conventional diazo salt. The plate is dried and exposed through a photographic negative. The exposed image areas decompose and become insoluble. The unexposed, nonimage areas remain soluble. The plate is developed with a lithographic lacquer which consists of two phases-- one phase containing an oleophilic resin and the other phase a hydrophilic gum. Upon application the oleophilic resin attaches to the exposed insoluble areas, while the hydrophilic phase cleans and protects the unexposed soluble nonimage areas. In this way the image areas are made oleophilic or ink-receptive and the nonimage areas are made hydrophilic or ink-repellent.
A positive plate is generally one upon which the non-image area is the portion of the light-sensitive diazo compound exposed to light while the unexposed portion is either oleophilic or adapted to be converted by chemical reaction to a hardened oleophilic ink-receptive image area.
In coating a metallic plate with a light-sensitive material it is desirable to provide an adherent, hydrophilic, abrasion resistance surface. This is best achieved by anodizing the surface of the aluminum base followed by a silicating treatment. In addition, the maximum lattitude between the oleophilic image areas and hydrophilic non-image areas is achieved.
Anodized aluminum bases for lithographic printing plates are well known and commercially available. Such plates are described in U.S. Pat. No. 3,181,461 issued May 4, 1965.
Prior to anodizing, it is common practice to grain the surface of the aluminum to increase its surface area. Graining can be carried out by mechanically treating the aluminum, for example by brush graining or ball grained or it can be grained chemically or electrochemically. Slurry brush graining has grown in importance over the past 20 years and today approximately 75% of the lithographic plates produced in the U.S. are grained using this technique.
Traditionally brush graining has been achieved by incorporating pumice or quartz into an aqueous slurry. These conventional abrasives are blocky and/or angular in shape thus presenting cutting edges for gouging or roughening surfaces in a random, nonuniform fashion. ("Brush Graining of Aluminum for Lithographic Printing Plates", J. H. Manhart, Alcoa). See also U.S. Pat. No. 3,891,516 issued June 24, 1975.
The ultimate test of the efficiency of graining is the quality of printing and the useful life of the printing plate. A good grain holds the organic coating in the image area and it also holds more water in the non-image areas making the water balance on the press less critical.